Hi kapi2454,
it is easy to do.
Your Benbox Version is 3.7.99 and windows is 664bit?
If so goto installdir (C:\Program Files\Benbox\3.7.99\roms)
or a similar path...
Here you can find the rom aka firmware for your spare arduino.
Best of them are something with LX-Nano.328p.hex in the name, if you using nano328p
As in other threat on this forum decribed...
Use "XLoader" (google XLoader and download) to transfer the LX-Nano.hex into your Arduino.
Try connection speed 115000 and if it's not working, try 56000.
After success, you can use your benbox-software.